1. Field of the Invention
The present invention relates generally to printers such as those used in conjunction with desk top computers for example, and in particular to an apparatus and method for reliably picking a sheet from a paper stack to feeding it to a position for being printed upon by the printer.
2. Description of Related Art
Printers designed for use with word processors or general purpose computers must be capable of printing on various sizes of paper. Accordingly, printers are often designed to receive sheets of paper from one of a number of interchangeable, removable paper trays, each of which is sized to hold a particular size of paper. It naturally follows that the printer must include a mechanism for removing a sheet of paper from atop a stack held in the tray and for transporting the sheet to the required position for being printed upon. As is common among those skilled in the art, this process is referred to hereinafter as picking a sheet from the stack.
In order for the printer to accurately print text or images in the desired location on a sheet, the sheet must be accurately and consistently positioned relative to the print head. Stated another way, the picking process must be highly accurate and reproducible by delivering the sheet to exactly the same position and in the proper orientation each time. In order to do so, the sheet must be picked from the stack in a highly predictable and consistent manner. Picking mechanisms typically include a rotatable drive shaft positioned above the stack and transversely to the direction of paper travel. Two pick rollers are mounted on the drive shaft for engaging the top sheet near each longitudinal side. A pick roller is usually made from a material such as soft plastic or rubber, and includes a cam like raised surface for frictionally engaging and advancing the top sheet as the drive shaft is rotated.
Variability in the picking process takes two principal forms, top of sheet variation and skewing, each of which may arise in several ways. Top of sheet variation refers to variation in the distance a sheet is transported during picking, resulting in variation of the position of the top of the sheet relative to the printing head. Top of sheet variation may result from inconsistent engagement of the pick roller with the top sheet owing to relatively wide manufacturing tolerances or deflection of the relatively soft materials of construction. Alternatively, as shown in FIG. 1, top of sheet variability "d" may result from the change in position and/or orientation of the stack in the tray may vary as the stack is depleted, resulting in variation in the timing of engagement of the picking mechanism.
Skewing refers to the sheet being rotated during picking, and usually results from variations in the timing of engagement between the pick rollers. Tighter control of manufacturing tolerances of pick rollers reduces skewing, but cannot eliminate it entirely due to the flexible nature of the materials of construction. Skewing may be reduced in cases where the stack is supported in the tray with one end raised by a process known as gravity dancing. The top sheet of the stack is engaged at its lower end by the pick rollers and fed along a downwardly angled path and engaged by the rotating dry rollers. The dry rollers are then rotated in reverse, moving the sheet upwardly until the sheet is clear of the dry rollers. As the dry rollers continue to rotate in reverse, the sheet is straightened relative to the dry rollers by being momentarily supported against the force of gravity with its lower edge positioned at the point where the dry rollers contact an underlying platten. The dry rollers are then rotated in the forward direction, advancing the paper to the printing position. While effective, this method requires an upwardly angled paper tray, and a relatively complex picking control algorithm.
An additional problem encountered in picking is multiple picks, that is the picking of sheets underlying the top sheet. Multiple picks may occur when underlying sheets in the stack are partially "dragged" out of the tray by the picking of sheets above, and is not suitably remedied by known picking mechanisms.
A need therefore exists for an improved picking mechanism which serves to accurately and consistently position successive sheets picked from atop a paper stack for printing, and which can be economically incorporated into known printer designs.